Exogenous retroviruses were analyzed for their influences on T cell repertoire. A defective murine leukemia virus which causes a mouse acquired immune deficiency syndrome (MAIDS) induced superantigen-like T cell activation in vitro. In vivo, this virus selectively activated and expanded CD4+ T cells expressing Va5, followed later in the course of infection by widespread immune deficiency in all T cells. The effect of milk-borne MMTV on the T cell receptor (TCR) repertoire was analyzed. A previously uncharacterized tumorigenic milk-borne virus in BALB/c mice (the BALB/cV virus) was found to induce deletion of T cells expressing TCR Va2 in developing mice. The roles of MHC class II, TCR, and CD28 costimulatory molecules in susceptibility to MMTV infection were tested. Milk-borne virus induced Va-specific deletion only in strains of mice bearing natural or transgenic I-E class II major histocompatibility complex (MHC) product. Moreover, susceptibility to milk-borne virus as determined by assays of viral pp28 or LTR mRNA was also dependent upon I-E expression. These findings indicate that viral infection is dependent upon superantigenic stimulation of host lymphoid cells. Studies employing CD28-deficient mice indicated that CD28- dependent costimulus plays a role in Va-specific T and B cell responses to in vivo challenge of adult mice with infectious MMTV. In contrast, Va-specific deletion in response to neonatal challenge, as well as susceptibility to infectious milk-borne virus, appeared to be CD28- independent.